Over recent years, the field of optoelectronic devices has developed rapidly, generating new and improved devices that go some way to meeting the ever increasing global demand for low-carbon emissions. However, this demand cannot be met with the devices currently available. The issues with the currently-available technology are illustrated below, using the area of photovoltaic devices.
The leading emerging technologies pushing to realise the ultimate goal of low cost solar power generation are dye-sensitized and organic photovoltaics. Dye-sensitized solar cells are composed of a mesoporous n-type metal oxide photoanode, sensitized with organic or metal complex dye and infiltrated with a redox active electrolyte. [O'Regan, B. and M. Gratzel (1991). “A Low-Cost, High-Efficiency Solar-Cell Based On Dye-Sensitized Colloidal TiO2 Films.” Nature 353(6346): 737-740.] They currently have certified power conversion efficiencies of 11.4% [Martin A. Green et al. Prog. Photovolt: Res. Appl. 2011; 19:565-572] and highest reported efficiencies are 12.3% [Aswani Yella, et al. Science 334, 629 (2011)]. The current embodiment of organic solar cells, is a nanostructured composite of a light absorbing and hole-transporting polymer blended with a fullerene derivative acting as the n-type semiconductor and electron acceptor [Yu, G., J. Gao, et al. (1995) Science 270(5243): 1789-1791 and Halls, J. J. M., C. A. Walsh, et al. (1995) Nature 376(6540): 498-500]. The most efficient organic solar cells are now just over 10% [Green, M. A., K. Emery, et al. (2012). “Solar cell efficiency tables (version 39).” Progress in Photovoltaics 20(1): 12-20]. Beyond organic materials and dyes, there has been growing activity in the development of solution processable inorganic semiconductors for thin-film solar cells. Specific interest has emerged in colloidal quantum dots, which now have verified efficiencies of over 5%, [Tang, J, et al. Nature Materials 10, 765-771 (2011)] and in cheaply processable thin film semiconductors grown from solution such as copper zinc tin sulphide selenide (CZTSS) which has generated a lot of excitement recently by breaking the 10% efficiency barrier in a low cost fabrication route. [Green, M. A., K. Emery, et al. (2012). “Solar cell efficiency tables (version 39).” Progress in Photovoltaics 20(1): 12-20] The main issue currently with CZTSS system is that it is processed with hydrazine, a highly explosive reducing agent [Teodor K. Todorov et al. Adv. Matter 2010, 22, E156-E159].
For a solar cell to be efficient, the first requirement is that it absorbs most of the sun light over the visible to near infrared region (300 to 900 nm), and converts the light effectively to charge. Beyond this however, the charge needs to be collected at a high voltage in order to do useful work, and it is the generation of a high voltage with suitable current that is the most challenging aspect for the emerging solar technologies. A simple measure of how effective a solar cell is at generating voltage from the light it absorbs, is the difference energy between the optical band gap of the absorber and the open-circuit voltage generated by the solar cell under standard AM1.5G 100 mWcm−2 solar illumination [H J Snaith et al. Adv. Func. Matter 2009, 19, 1-7]. For instance, for the most efficient single junction GaAs solar cells the open circuit voltage is 1.11 V and the band gap is 1.38 eV giving a “loss-in-potential” of approximately 270 meV [Martin A. Green et al. Prog. Photovolt: Res. Appl. 2011; 19:565-572]. For dye-sensitized and organic solar technologies these losses are usually on the order of 0.65 to 0.8 eV. The reason for the larger losses in the organics is due to a number of factors. Organic semiconductors used in photovoltaics are generally hindered by the formation of tightly bound excitons due to their low dielectric constants. In order to obtain effective charge separation after photoexcitation, the semiconducting polymer is blended with an electron accepting molecule, typically a fullerene derivative, which enables charge separation. However, in doing so, a significant loss in energy is required to do the work of separating the electron and hole. [Dennler, G., M. C. Scharber, et al. (2009). “Polymer-Fullerene Bulk-Heterojunction Solar Cells.” Advanced Materials 21(13): 1323-1338] Dye-sensitized solar cells have losses, both due to electron transfer from the dye (the absorber) into the TiO2 which requires a certain “driving force” and due to dye regeneration from the electrolyte which requires an “over potential”. For dye-sensitized solar cells, moving from a multi-electron Iodide/triiodide redox couple to one-electron outer-sphere redox couples, such as a cobalt complexes or a solid-state hole-conductor, improves the issue but large losses still remain [Oregan 91, Aswani Yella, et al. Science 334, 629 (2011), and Bach 98 and Gratzel solid-state JACS]. There is an emerging area of “extremely thin absorber” solar cells which are a variation on the solid-state dye-sensitized solar cell. [Y. Itzhaik, O. Niitsoo, M. Page, G. Hodes, J. Phys. Chem. C 113, 4254-4256 (2009)] An extremely thin absorber (ETA) (few nm thick) layer is coated upon the internal surface of a mesoporous TiO2 electrode, and subsequently contacted with a solid-state hole-conductor or electrolyte. These devices have achieved efficiencies of up to 7% for solid-state devices employing Sb2S3 as the absorber, [J. A. Chang et al., Nano Lett. 12, 1863-1867 (2012)] and up to 6.5% employing a lead-halide perovskite in photoelectrochemical solar cell. [A. Kojima, K. Teshima, Y. Shirai, T. Miyasaka, J. Am. Chem. Soc. 131, 6050-6051 (2009); J-H Im, C-R Lee, J-W Lee, S-W Park, N-G Park, Nanoscale 3, 4088-4093 (2011)] However, the ETA concept still suffer from rather low open-circuit voltages.
There is therefore a need for a new approach to developing optoelectronic devices. New systems that combine favourable properties such as high device efficiency and power conversion, with device stability are required. In addition, the devices should consist of inexpensive materials that may be easily tuned to provide the desirable properties and should be capable of being manufactured on a large scale.